1. Field of the Invention
This invention relates to a process and apparatus for transfer of liquefied natural gas, such as for refueling vehicles with liquefied natural gas. The process provides direct transfer from a liquefied natural gas storage tank to a receiving tank, such as a vehicle tank, using liquid nitrogen to both subcool liquefied natural gas in the storage tank and act as a direct pressurant for transport of the liquefied natural gas from the storage tank to the receiving tank.
2. Description of Related Art
With the increased potential for use of liquefied natural gas (LNG) as a vehicle fuel, there is a need for easier and more efficient transfer of LNG, particularly for easy vehicle refueling. LNG is normally stored as a saturated liquid, that is, at its boiling point. Therefore, if any heat is transferred to the LNG during transfer, vaporization results. The boiling temperature of LNG decreases with pressure reduction resulting in vaporization of saturated LNG when transferred from a higher pressure tank to a lower pressure tank. Also, the density of LNG decreases as the pressure increases, resulting in the requirement of a larger tank to contain a given mass of saturated LNG when its pressure is increased.
Pump systems have been used in refueling to transfer LNG from a storage tank to a vehicle tank. In-line pumps, usually centrifugal, have been used for saturated LNG transfer with pump cavitation being a usual problem. In-line pump systems for vehicle refueling usually have a cool-down stage in which the initial LNG passed through the system is used to cool the pump and piping prior to initiation of the vehicle tank filling stage. During the cool-down stage LNG is vaporized, with the vaporized natural gas usually vented to the atmosphere. Pumps submerged in the LNG storage tank or in a separate sump tank result in less LNG vaporization, since they are maintained at the LNG storage tank temperature, but they are inconvenient with respect to hardware and accessibility. Submerged pumps also increase heat transfer to the storage LNG increasing the boil-off rate and during transfer vaporization still occurs as piping downstream of the pump is cooled.
Pressure transfer is used to transfer LNG from tank trucks to storage tanks by vaporization of LNG into the ullage to build up pressure to effect the liquid LNG transfer with the LNG vapor being vented upon completion of the transfer. The same general method can be used to transfer LNG from a refueling facility tank to a vehicle tank. Prior to each LNG vehicle refueling, pressure can be built up in the refueling facility primary LNG storage tank or in a smaller secondary tank which is supplied with LNG from the primary tank. In either case, considerable gas venting occurs when the pressure is relieved subsequent to a vehicle refueling cycle. Alternatively, pressure can be maintained continuously in the primary storage tank, in which case LNG becomes saturated at the higher pressure thereby increasing vaporization when transferred to a lower pressure tank. In all cases, vaporization also occurs as LNG initially cools the refueling line.
All of the above described LNG refueling systems result in significant LNG vaporization. It is undesirable to vent natural gas vapor since it contributes to global warming by depleting the ozone layer and, further, such venting results in less fuel being transferred to the vehicle tank. It is generally impractical to reliquefy the vaporized gas at the refueling site. Also, vaporization of LNG results in undesirable enrichment of the remaining LNG with heavier hydrocarbons rendering the LNG unacceptable for some engines.
Liquefaction of methane and natural gas is well known. Liquefaction of methane by use of initially liquid nitrogen in a refrigeration cycle is taught by U.S. Pat. No. 2,909,906. Liquefaction of natural gas by heat exchange with a nitrogen refrigeration cycle is taught by U.S. Pat. No. 3,780,534. Transport of methane as a liquid using liquefied nitrogen as a heat exchange material at each end of the transport, with liquid methane and liquid nitrogen being transported in opposite directions, is taught by U.S. Pat. Nos. 2,975,604 and 3,018,632.
Filling of high pressure gaseous fuel tanks rapidly by first pressurizing the tank with a predetermined quantity of cryogenic liquid, such as LNG or nitrogen, to achieve the desired pressure is taught by U.S. Pat. No. 5,211,021. Refueling compressed natural gas vehicle tanks by vaporizing LNG in a storage tank, on demand delivery basis, to high pressure natural gas vapor by a heat exchanger or external heat source is taught by U.S. Pat. No. 5,107,906.
Filling of vehicle refrigeration tanks with low pressure liquid carbon dioxide using carbon dioxide snow to condense carbon dioxide vapor created as a result of the transfer is taught by U.S. Pat. No. 4,100,759. Filling of liquid hydrogen vehicle fuel tanks wherein the delivery lines are cooled prior to connection with the vehicle by passage of liquid hydrogen is taught by U.S. Pat. No. 4,608,830. Avoidance of pressure build-up in vehicle LNG fuel tanks due to heat transfer over time when fuel is not being consumed and during delivery by cooling with liquid nitrogen through indirect heat transfer from a refrigeration cycle is taught by U.S. Pat. No. 4,292,062.
A liquid propellant, such as hydrogen, is maintained at desired pressure and temperature in a tank by pressurized gas, such as helium, from a second tank being discharged into the liquid in the propellant tank with the entire system being cooled by evaporation of the propellant and indirect heat exchange with both tanks is taught by U.S. Pat. No. 3,473,343. Transfer of liquid fuels, such as methane, by vaporizing a cryogenic liquid, such as helium, with the vapor being used as a pressurant is taught by U.S. Pat. No. 3,803,858. The vaporization taught by the '858 patent may be effected by indirect heat exchange with the liquid fuel or by discharge into the liquid fuel.
U.S. Pat. No. 5,121,609 teaches refueling LNG vehicles using a pressure building tank, separate from the LNG storage tank, wherein increased pressure is achieved by vaporization of LNG into the ullage to act as the pressurant for liquid transfer and decreased pressure is achieved by liquid nitrogen in indirect heat exchange through a heat exchanger to condense vaporized natural gas in the ullage. The '609 patent teaches that for deliveries of less than 10 gallons, a low quantity use line may be used which sub-cools LNG by passage through a heat exchanger in heat exchange with liquid nitrogen or suitable condensing agent or mechanism. The '609 patent teaches venting of natural gas, which forms by vaporization in the ullage of the vehicle tank, back to the ullage of the fueling station may be achieved by control of pressure in the fueling station and the vehicle fuel tank.